kollesco since you already switched the main board and the battery, I would check in the back casing with the antennas etc. Your charging port which is commonly the source of this is part of the main board so it should not be the issue. Some people say if they sign out of their Samsung accounts, clear the phones cache and get into safe mode, that this message stopped from appearing. Just make sure you have all of your files/password etc. before attempting this.
== How to Disable Moisture Detected Error ==
If the moisture detected error won’t go away even after cleaning and wiping the USB port, and you are still unable to charge your Samsung device, try the following things.
# ***Use a Wireless charger***: Get a wireless charger so that your Samsung device does not run out of battery until the moisture has been detected issue persists.
# ***Turn off the device***: Power off your Samsung phone and plug in the charging pin. Your device will start charging.
# ***Force stop Android System***: Open device ***Settings*** and type ‘***Android System***‘ in the search box. Tap on ***Android System*** to open the app info page and then tap on the ***Force stop*** option. Now, try charging the device.
# ***Disable Fast Charging:*** Disabling fast charging can make the moisture-detected warning on your Samsung Galaxy device go away. The steps to reach the setting differs depending on the software version.
** Go to ***Settings> Device maintenance > Battery > 3-Dot icon > Advanced settings > Fast charging*** and turn it off.
# Go to ***Settings> Device care > Battery > Charging*** and disable ***Fast charging***
from [https://technastic.com/fix-moisture-detected-error-samsung/|here]